build a frame from 2x4s for a box as large as you can manage in your shed
next, panel the inside of the box with plywood
now, cover the plywood with some 20-40 mil. pond liner, trim it so it over hangs your box edges about 8"
using some scrap wood, wrap the edges of the liner around a few times, and screw them into the edges of the box
fill with water, connect your filters, and you're done
the question I've got however, you mention a *shed* ... is this shed heated?
it's very likely that it will get colder in your shed then it will in the ground that your pond is dug into.
how about covering your existing pond with a tarp (leave some air holes) and using a 100watt bulb a heater to keep it from icing over, along with a pump to keep the oxygen circulating.
water movement alone will not stop water from freezing, but it will make sure to circulate warmer water from the bottom of the pond with colder water from the top... unless you've got a very inefficient or massive pump, the heat it gives off will not protect the pump and the pond from freezing.